1. Field of the Invention
The present invention relates to the thin film semiconductor device which may be used preferably for display, image scanner, etc., in particular the thin film semiconductor wherein uniformity and reliability of electric characteristics may be improved even when the area of thin film transistor and thin film transistor type optical sensor is enlarged.
2. Related Background Art
In recent years, with the progress of office automation, input-output devices such as display, image scanner, etc. are regarded to be important elements as the man-machine interface of OA apparatus such as word processor, personal computer facsimile, etc. and there is a demand for light-weight, thin and low priced elements.
From such viewpoints, there has been development of the active matrix type liquid crystal display which is obtained by forming a thin film semiconductor. Examples of the development are hydrogenated amorphous silicon, polysilicon, etc. on an insulation substrate of large areas and thereby forming plural number of film transistors or the photoelectric conversion device having optical sensor composed of the said thin film transistor.
FIG. 1A is a schematic longitudinal sectional view of an example of the construction of the conventional thin film transistor.
Here, the gate insulation layer 2 is deposited on the gate electrode 1 and a film semiconductor layer 4 which becomes the channel, for example, hydrogenated amorphous silicon (hereinafter called a-Si:H) layer is provided thereon.
Further, n+ layer 6 is provided between the metal electrodes of source-drain electrodes 7 and 8 and by forming a junction which takes ohmic characteristics to electrons and blocking characteristics to the hole, the said n+ layer functions as n channel transistor. The surface of the thin film semiconductor 18 is at the upper plane of the channel section.
FIG. 1B is the plan view of FIG. 1A. FIG. 1B indicates in particular the thin film transistor having a planar type electrode construction (in FIG. 1B, electrodes 7 and 8 are comb type) which has been proposed for increasing channel length and solving the problem in the process.
The thin film transistor of FIGS. 1A and 1B may be utilized as the optical sensor of secondary photocurrent type. (For example, Japanese Patent Appln. Laid-Open No. 60-101940). Hereinafter such an optical sensor shall be called thin film transistor type optical sensor.
FIG. 2 is a schematic longitudinal sectional view of an example of the construction conventional co-planar type optical sensor which is a secondary photocurrent type sensor. An optical sensor indicated in FIG. 2 has substantially the same construction as the thin film transistor explained in FIG. 1 except that it has no gate electrode 1 and functions as the secondary photocurrent type optical sensor.
FIGS. 3A and 3B are the process drawings to show the manufacturing method of the conventional thin film transistor of FIG. 1.
The method of manufacture of such thin film transistor is disclosed, for example, in Japanese Patent Appln. Laid-Open No. 63-9157.
In FIG. 3A, G is a glass substrate and 1 is the Cr which forms gate electrode. After selectively forming gate electrode 1, silicon nitride film which becomes the gate insulation layer 2 is deposited for 3000 .ANG., a-Si:H4 which becomes thin film semiconductor layer 4 is deposited for 5000 .ANG. and n+ layer 6 is continuously deposited for 1500 .ANG. on the substrate by plasma CVD. Further, an aluminum layer which becomes source-drain electrodes 7 and 8 is deposited by sputtering etc. Thereafter, photosensitive resin 10 is coated all over the surface and it is exposed and patterned. FIG. 3B indicates the state after patterning of the aluminium which forms source-drain electrodes (i.e., the state after forming the source-drain electrodes 7 and 8). At this time, photosensitive resin 10 exists on the electrode. Using this photosensitive resin 10 as a mask, n+layer 6 is etched, for example, by RIE for the specified depth and thereafter photosensitive resin is peeled off. Then the thin film transistor is separated between devices and the thin film transistor shown in FIG. 1A is obtained.
The surface of the thin semiconductor film of the thin film transistor is easily affected by the atmosphere and if oxygen gas or steam is directly adsorbed or diffused to such surface, its electric characteristics greatly change because the semiconductor film is extremely thin. Because of that, studies are being made concerning the method to cover the surface of the device by a protective film made of silicon nitride (Si3N4) or aluminum oxide (A1203) after the aforesaid process. (For example, Japanese Patent Appln. Laid-Open No. 59-61964).
A method is also proposed in which the polyimide resin film polymerized by heat treatment is used as the protective film.
Also, to give more stability, a proposal has been presented to form the second protective layer of the same material as that of film semiconductor 4 on the polymerized polyimide film. (For example, Japanese Patent Appln. Laid-Open No. 1-137674).
Generally speaking, a plural number of film transistors or a plural number of optical sensors which have been formed with thin film transistors are required to have uniform characteristics within the substrate even when it is large in area size. However, the thin film transistor or optical sensor formed in the step of FIGS. 3A and 3B tend to lack uniformity of electric characteristics when, for example, RIE (reactive ion etching) is employed particularly in the etching of the n+ layer of FIG. 3B. For example, when a large number of thin film transistors are formed on the substrate, the threshold voltage which determines the performance characteristics of thin films transistor produces a distribution of several volts in the substrate and it presents a serious obstacle in the use of the thin film transistor. As a result, the picture quality substantially changes in the case of active matrix type display. In the case of the optical sensor, the characteristics of photocurrent and dark current which are the basis characteristics of the sensor vary considerably from one optical sensor to another and it results in a serious deterioration of the quality of read-out image.
Especially when the protective film on the thin film transistor or optical sensor with non-uniform characteristics is made or organic material such as polyimide, there are occasions when environmental stability such as moisture resistance can not be expected.
On the other hand even when the protective film is made of inorganic material (for example, a-SiNx:H), the distribution of characteristics or undesirable electric characteristics as stated above may sometimes result, depending on the method of formation of the protective film taken or the composition of the formed protective film. It is also known from the report of Hiranaka et al. who have viewed the relation between the composition of insulation layer and film semiconductor layer 4, that there is the problem of gate interface of the thin film transistor. Hiranaka et al. suggest that the problem exists of gate interface between gate insulation film 2(SiNx:H) and the thin film semiconductor layer 4(a-Si:H) and the composition of the gate insulation film seriously influences the band conditions of film semiconductor layer 4 J. Appl. Phys. 62(5), P2129(1987) and J. Appl. Phys. 60(12), p4294(1986) }. Moisture resistance is also largely dependent upon the composition of insulation layer used as the protective film.